An array of critical flow venturis (CFVs) was installed in a blow-down facility and used to calibrate a large (i.e., D = 89.5 cm) 8 path ultrasonic flow meter (USM) at atmospheric pressure. The calibration was performed in dry air over a flow range from 2 kg/s to 36 kg/s. At the largest flows, pressure transients associated with the blow-down system resulted in quasi-steady flow conditions. The quasi-steady conditions were measured using a high speed data acquisition system that scanned 66 instruments (i.e., pressure and temperature sensors) every 1.8 s. Corrections were made for thermal lag of the RTD temperature sensors, and for mass storage effects in the 230.3 m3 connecting volume between the CFV array and the USM. This paper presents the results of the calibration and the corresponding uncertainty analysis.